In the production of petrochemicals, water is often used to control various chemical reactions, for example by transferring heat from process streams to quench reactions. When such water is subjected to intimate contact with a process stream, it is commonly referred to as process water. In ethylene manufacturing plants, steam is contacted with the process feedstock to control the pyrolysis (cracking) process by lowering the partial pressure of the hydrocarbon feedstock, improving the efficiency of the conversion reaction. Downstream of the pyrolysis reactors, a water quench tower is further employed to cool the gas leaving a primary fractionator or a transfer line exchanger.
In the base of the quench water tower or in a discrete water/oil separator, hot process water is separated from the hydrocarbon products that condense along with steam. These condensed hydrocarbon products are commonly referred to as pyrolysis gasoline, or pygas. The low-density, liquid hydrocarbons that comprise pygas rise to the top surface of the bulk liquid, where they are drawn off via a weir system and typically become part of a recycle stream or byproduct stream for export or further processing. The bulk phase is water containing both emulsified and dissolved light liquid hydrocarbons derived from the pygas. Once the emulsified hydrocarbons are substantially separated from the process water, the water can be recycled for use in the cracking and quenching process, while the pygas component can e.g. be returned to the primary fractionator as reflux or further processed for some other purpose. Ethylene plants that crack gaseous feedstock are generally constructed without a “primary fractionator” and in the absence of this process vessel there is an additional hydrocarbon condensate fraction that must be addressed. This other hydrocarbon fraction has been called “heavy pygas”, “heavy hydrocarbon”, or “pytar” (pyrolysis tar). This hydrocarbon fraction is denser than the process water and there are mechanical provisions for removing this fraction from the bottom of the oil/water separator.
Severe process conditions exist in the quench water tower system. Temperatures at the base of the quench tower can approach 100° C., for example about 60° C. to 100° C., for example about 80° C. to 90° C. Additionally, corrosion and fouling potential due to the presence of organic acids and oxygenates, polymerization potential, heat removal by “pumparounds”, and a mixture of hydrogen, steam, and a mixture of pygas and in some embodiments also pytar are present during operation of these towers and contribute to the formation of the subject pygas emulsions.
Pygas and various other hydrocarbon contaminants can concentrate in the process water. Other contaminants include organic acids such as acetic, formic, propionic, and/or butyric acid that are formed during the cracking process. These are readily dissolved by the process water, providing a water phase having a pH of about 5-6 or less. Nevertheless, pH of 8-9 can also be encountered due to the presence of alkaline species, such as ammonia in the process water. If unchecked, the combined emulsified and dissolved compounds can lead to fouling, foaming, corrosion and product quality issues downstream and also upstream, due to the use of recycled process water, as quench water.
However, pygas or a portion thereof often becomes emulsified in the process water or a portion thereof, a problem familiar to those of skill in ethylene manufacturing. Such emulsions are difficult to address due to the harsh conditions present at the oil/water separator.
In ethylene production, the “Dilution Steam System” typically consists of a quench tower, an oil/water separator, a process water stripper to remove dissolved hydrocarbons, and a dilution steam generator. Steam from the dilution steam generator is sent to the pyrolysis furnace and is recovered as water in the quench tower. This complex water loop can experience a variety of problems due to process water contaminants. The inability to resolve, or demulsify, these pygas/process water emulsions results in an increase of fouling in the dilution steam generator and quench water exchangers, as well as increased stripping steam requirements in the process water stripper. The mixture of hydrocarbons found in a particular sampling of pygas will vary as a consequence of plant design, hydrocarbon feedstock, and plant operating conditions. For example, a plant designed to crack liquid naphtha feedstock will have a primary fractionator that captures and fractionates hydrocarbons with high boiling points, while steam and pygas vapors are passed through to the water quench tower where they are condensed; if the operation of the primary fractionator is changed, which results in a pygas with a higher distillation endpoint, the ethylene producer will typically be challenged by more stable emulsions due to the altered composition of the pygas.
A different example would be a plant designed to crack gaseous feedstocks; such a plant is designed to operate without a primary fractionator and accordingly, the oil/water separation unit is designed to remove a heavier-than-water hydrocarbon phase, as well as a lighter-than-water hydrocarbon phase. If such a plant is operated at high cracking severity, that is, at a higher temperature during pyrolysis to maximize ethylene yield, the proportion of pytar increases, which increases the likelihood of pytar droplets becoming entrained into the pygas fraction. It has been observed that increasing the pytar content of the pygas increases emulsion stability of the pygas-pytar mixture.
By way of example, in many cases the process water contains not only traces of styrene but also oligomers of styrene. Capturing styrene in the quench tower recycling loop exacerbates fouling by inducing its polymerization, leading eventually to deposition of the styrenic products on equipment surfaces.
The pygas emulsions with process water are distinct and different from those that are found in oil field produced water. Oil field produced waters contains asphaltenes, resins and very long chain paraffins. These hydrocarbon types are almost never found in ethylene quench waters.
There is a need in the industry to resolve, or demulsify, emulsions formed by pygas and process water within ethylene manufacturing plants. There is a need in the industry to resolve such emulsions quickly and using simple methodology.